1. Field of the Invention
This invention relates to a tracking control system included in a system for recording an information signal by forming many parallel tracks on a recording medium and for reproducing the information signal from the medium.
2. Description of the Related Art
The systems of the above stated kind have recently come to be often arranged to record and reproduce various information signals by digitizing them. The frequency band of the digitized information signals is much wider than that of analog information signals. The wider band necessitates high density magnetic recording or the like and thus, for example, does not allow the system to form the recording tracks at a wide pitch.
In reproducing the digital information signal, on the other hand, the probability of occurrence of a code error greatly increases in the event of a tracking error. The code error may be correctable by means of a so-called error correction code or the like. However, it is not desirable to increase the correcting power of the error correction code as it increases redundancy. It is, therefore, an important theme for the systems of this kind to enhance the tracking precision thereof.
Meanwhile, typical tracking control methods which have heretofore been employed for the system of this kind include a method called a CTL method. In accordance with this tracking control method, a control signal (hereinafter referred to as a CTL signal) which is of a frequency corresponding to the track pitch is recorded at an edge part of a tape-shaped recording medium by means of a fixed head; and a tracking error signal is obtained by reproducing the CTL signal and by comparing the phase of the reproduced CTL signal with that of a reference signal. In another tracking control method which is called a four frequency method, pilot signals of four different frequencies are superimposed on information signals and recorded on the tape-shaped recording medium one after another, one in each of tracks; and, during a reproducing operation, a tracking error signal is obtained by comparing the levels of the pilot signals reproduced from two tracks adjacently located on both sides of a track which is mainly under control.
In the case of the CTL method, the tracking control is not accomplished on the basis of information obtained from the recording tracks. In the event of impaired linearity of the tracks or a mounting error of a CTL signal reproducing head, therefore, it becomes impossible to obtain an accurate tracking control signal by this method.
The four frequency method solves the problem presented by the CTL method. In the four frequency method, however, the need to superimpose as many as four different kinds (frequencies) of pilot signals upon the information signal imposes a limit on the frequency band of the information signal. In the case of a digital signal in particular, it is impossible to have the pilot signals of as many as four different frequencies superimposed on the signal as the lower end of the recording band of the digital signal comes as low as 100 KHz or thereabout.
In recording digital video signal or a wide-band video signal having a large amount of information per unit time, for example, it becomes difficult to record it in a single channel. In such a case, therefore, it has been practiced, for example, to record one field portion of the video signal by dividing it into a plurality of tracks through a multi-channel arrangement. Such a recording system then necessitates a reproducing system to make a discrimination as to which of the tracks has the record of one of the video signal channels. If that discrimination is not possible, the divided video signal cannot be recombined. To obviate the necessity of various switching circuits for discrimination of recorded signal channels, it is preferable to use an n number of reproducing heads in reproducing a video signal divided into n channels.
In that case, the n number of reproducing heads must be controlled to have them accurately and reliably trace only the track that has the video signal of an applicable channel recorded therein. Further, in case that the number of the heads is n/i (i: an integer), each of the heads must be arranged to trace a specific track allotted to the applicable signal channel to be reproduced so long as a plurality of reproducing heads are in use.
This requirement can be met, for example, by recording the CTL signal in a cycle of n track pitches according to the above stated CTL method. Whereas, in the case of the four frequency method, the method meets the requirement only in the event of n=4 or 2.
In case that a VTR is arranged to record a digital signal by simultaneously using a plurality of heads, there arises the following problems:
FIGS. 1 and 2 of the accompanying drawings illustrate the problems of the prior art. FIG. 1 shows the head arrangement of a VTR which is arranged to use a plurality of heads at the same time. FIG. 2 shows the tracing loci of the heads obtained on a magnetic tape employed as a recording medium. Referring to FIG. 1, rotary heads h1, h2, h3, h4, h5 and h6 are mounted on a rotary cylinder D at a phase difference of 60 degree from each other. These heads are disposed in the same position in the direction of the rotation axis of the rotary cylinder D. They are arranged to trace the surface of the magnetic tape T as shown in FIG. 2. The tape T is wrapped at least 180 degrees round the circumferential surface of the rotary cylinder D by means of tape guide posts g1 and g2.
In respect of tracking control to be accomplished by using a signal reproduced by the rotary head with the VTR arranged as described above, it is generally practiced to use only the signals that are reproduced by some of the plurality of heads which are simultaneously tracing the tape. Assuming that signals reproduced by the heads h2 and h5 are to be used for tracking control among the heads h1 to h6 of FIG. 1, for example, it is not always possible to have other heads accurately trace applicable tracks because of head mounting errors, even if the tracking control is perfectly accomplished. The head mounting errors tend to result in faulty tracing positions as indicated by broken lines in FIG. 2.
Assuming that the heads h1, h2 and h3 are now in tracing positions as shown by the broken lines in FIG. 2, in accordance with the above stated method for tracking control, the travel of the magnetic tape is controlled in such a way as to cause them to trace the parts of the tape as shown by full lines in FIG. 2. In that instance, however, the tracking control causes the head h3 to further deviate from a track tr3 which is to be traced. In the case of an analog signal, the head 3 may still be able to give some amount of a reproduced signal if the signal is azimuth recorded. In reproducing a digital signal, however, the reproduction of the signal becomes hardly possible as the code error would be increased by the tracking control. A digital signal permits a discrimination between "1" and "0" if a reproduced signal is obtainable at a certain level. Then, with an error correcting process, etc., carried out, the reproduced signal can be obtained in a nearly integral state. In reproducing a digital signal, therefore, the positions shown by the broken lines in FIG. 2 are better than the positions shown by the full lines in terms of tracking control. Whereas, the above stated tracking control method causes the head to be shifted from the broken line position to the full line position in such a manner that is contrary to the above stated principle of digital signal reproduction.